The long term objective of this project is to understand how CD4 and CD8 function in T cell development at the molecular and cellular level. Although much has been learned in recent years about how CD4 and CD8 participate in T cell recognition, the role of these receptors in T cell development is not well understood. We have begun to explore this question by modifying the activity of CD8 and CD4 in vivo. Our approach is to introduce altered forms of CD8 and CD4 in developing thymocytes using transgenic mice. The experiments proposed here will test the hypothesis that intracellular signals mediated by CD4 and CD8 control the decision of an immature CD4+CD8+ thymocyte to become a CD4+CD8- T cell or a CD4-CD8+ T cell. If this hypothesis is correct, hybrid proteins consisting of the extracellular, recognition domain of CD8 and the cytoplasmic, signaling domain of CD4, should cause thymocytes bearing class I MHC specific TCRs to turn off the CD8 gene rather than the CD4 gene. If CD4 and CD8 intracellular signals also influence the choice of T cell effector function, the CD8/4 hybrid protein may induce class I specific T cells to produce a lymphokine pattern which is characteristic of class II specific CD4 cells. We will investigate these possibilities by examining the effect of CD8/4 hybrid molecules on thymic development and lymphokine production in transgenic mice. If CD4 and CD8 signals determine the commitment to a CD4 or CD8 lineage, this implies that CD4 and CD8 mediate distinct intracellular signals. However, both CD4 and CD8 associate with the tyrosine kinase, p56lck, and there is currently no evidence that they generate different signals. We will investigate the possibility that the beta chain of CD8 is involved in generating signals which control T cell development, by engineering dominant negative mutations of CD8 beta in transgenic mice, and by identify signaling molecules which interact with CD8 beta. This project is motivated by the belief that a basic knowledge of the workings of the normal mammalian immune system is critical to our understanding of diseases of the immune system, such as HIV, leukemia, and autoimmune disorders. This knowledge may also provide the basis for manipulating the immune system to improve the resistance to other diseases.